300 FRAGMENTS OF SCIENCE. 



No more striking or instructive illustration of the 

 influence of coincidence could be adduced than that fur- 

 nished by the radiation from a carbonic oxide flame. Here 

 the product of combustion is carbonic acid; and on the 

 radiation from this flame even the ordinary carbonic acid 

 of the atmosphere exerts a powerful effect. A quantity of 

 the gas, only one-thirtieth of an atmosphere in density, 

 contained in a polished brass tube four feet long, intercepts 

 50 per cent, of the radiation from the carbonic oxide flame. 

 For the heat emitted by lampblack, olefiant gas is a far 

 more powerful absorber than carbonic acid; in fact, for 

 such heat, with one exception, carbonic acid is the most 

 feeble absorber to be found among the compound gases. 

 Moreover, for the radiation from a hydrogen flame olefiant 

 gas possesses twice the absorbent power of carbonic acid, 

 while for the radiation from the carbonic oxide flame, at a 

 common pressure of one inch of mercury, the absorption 

 by carbonic acid is more than twice that of olefiant gas. 

 Thus we establish the coincidence of period between car- 

 bonic acid at a temperature of 20 degrees C. and carbonic 

 acid at a temperature of over 3,000 degrees C., the periods 

 of oscillation of both the incandescent and the cold gas 

 belonging to the ultra-red portion of the spectrum. 



It will be seen from the foregoing remarks and experi- 

 ments how impossible it is to determine the effect of tem- 

 perature pure and simple on the transmission of radiant 

 heat if different sources of heat be employed. Throughout 

 such an examination the same oscillating atoms ought to 

 be retained. This is done by heating a platinum spiral by 

 an electric current, the temperature meanwhile varying 

 between the widest possible limits. Their comparative 

 opacity to the ultra-red rays shows the general accord of 

 the oscillating periods of the vapors referred to at the com- 

 mencement of this lecture with those of the ultra-red un- 

 dulations. Hence, by gradually heating a platinum wire 

 from darkness up to whiteness, we ought gradually to 

 augment the discord between it and these vapors, and thus 

 augment the transmission. Experiment entirely confirms 

 this conclusion. Formic ether, for example, absorbs 45 

 per cent, of the radiation from a platinum spiral heated to 

 barely visible redness; 32 per cent, of the radiation from 

 the same spiral at a red heat; 26 per cent, of the radiation 

 from a white-hot spiral, and only 21 per cent, when the 



